The present invention relates to methods for controlling the quality of an industrial process, comprising the steps of:                providing one or more reference signals for an industrial process;        acquiring one or more real signals that are indicative of the quality of said industrial process; and        comparing said one or more reference signals with said one or more real signals in order to identify defects in said industrial process.        
Monitoring of the defects in industrial processes assumes increasing economic importance on account of its impact on the analysis of quality of industrial products. The possibility of obtaining an on-line and automatic assessment of the quality of an industrial process presents many advantages both from the economic point of view and from the standpoint of the speed of the process. Desirable characteristics of the system are hence:                on-line and real-time processing; and        capacity for accurate recognition of the main defects in production.        
Currently the problem of recognition of the quality of an industrial process, and consequently the identification of defects, is approached by an inspection carried out off line by skilled staff, or else using automatic methods, which, by means of sensors, identify only some of the defects listed above, in a way that is far from satisfactory and is moreover sensitive to the different settings of the machine.
There are known methods and systems for controlling the quality in industrial processes, for example applied to on-line monitoring of the laser-welding process, in particular in the case of welding of sheet metal. The control system is able to assess the presence of porosities in the welding area or else, in the case of butt-welded thin sheet metal, the presence of defects due to overlapping or poor jointing of the sheet metal.
The above used systems base quality control on a comparison between the signals detected during the process and one or more predetermined reference signals indicating a good-quality weld. Said reference signals, which usually range in number between two and ten, are arranged starting from a number of samples of good-quality welds. Obviously, said mode of procedure implies the presence of a skilled operator who is able to certify the goodness of the weld at the moment of creation of the reference signals, and involves expenditure in terms of time and sometimes also in terms of waste of material (used for making the samples necessary for obtaining the reference signals). In some cases there are also pre-arranged reference signals indicating a defective weld, this, however, involving additional problems and difficulties.
From the European patent application No. EP-A-1275464 filed in the name of the present applicant, it is known to split into blocks the signal acquired via photodiode, which collects the radiation emitted by a welding spot, calculating the mean of the signal in each block sampled and considering the blocks having a value smaller than or equal to the offset of the photodiode as indicating the presence of a defect. Said method eliminates the need for the reference signal; however, it enables only a very approximate detection of the defects.